The present invention relates to a method for erasing the image of a lithographic printing master by treating the printing surface with an atmospheric plasma.
Printing presses use a so-called master such as a printing plate which is mounted on a cylinder of the printing press. The master carries an image which is defined by the ink accepting areas of the printing surface and a print is obtained by applying ink to said surface and then transferring the ink from the master onto a substrate, which is typically a paper substrate. In conventional lithographic printing, ink as well as an aqueous fountain solution are fed to the printing surface of the master, which consists of oleophilic (i.e. ink accepting) and hydrophilic (water accepting) areas. In driographic printing, only ink is applied to the printing surface, which consists of ink accepting and ink repelling areas. These ink-repelling areas are often called oleophobic or ink-abhesive areas. Driographic plates are sometimes simply called xe2x80x98dryxe2x80x99 plates as distinct from the conventional xe2x80x98wetxe2x80x99 plates.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an imagesetter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
In recent years the so-called computer-to-plate method has gained a lot of interest. This method, also called direct-to-plate method, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. In the field of such computer-to-plate methods the following improvements are being studied presently:
(i) On-press imaging. A special type of a computer-to-plate process, involves the exposure of a plate precursor while being mounted on a plate cylinder of a printing press by means of an image-setter that is integrated in the press. This method may be called xe2x80x98computer-to-pressxe2x80x99 and printing presses with an integrated image-setter are sometimes called digital presses. A review of digital presses is given in the Proceedings of the Imaging Science and Technology""s 1997 International Conference on Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394 and EP-A 774 364. The best known imaging methods are based on ablation. A problem associated with ablative plates is the generation of debris which is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the integrated image-setter. Other methods require processing with chemicals which may damage the electronics and other devices of the press.
(ii) On-press coating. Whereas a plate precursor normally consists of a sheet-like support and one or more functional coatings, computer-to-press methods have been described wherein a composition, which is capable to form a printing surface upon image-wise exposure and optional processing, is provided directly on the surface of a plate cylinder of the press. EP-A 101 266 describes the coating of a hydrophobic layer directly on the hydrophilic surface of a plate cylinder. After removal of the non-printing areas by ablation, a master is obtained. U.S. Pat. No. 5,713,287 describes a computer-to-press method wherein a so-called switchable polymer such as tetrahydropyranyl methylmethacrylate is applied directly on the surface of a plate cylinder. The switchable polymer is converted from a first water-sensitive property to an opposite water-sensitive property by image-wise exposure. EP-A 802 457 describes a hybrid method wherein a functional coating is provided on a plate support that is mounted on a cylinder of a printing press.
(iii) Elimination of chemical processing. The development of functional coatings which require no processing or may be processed with plain water is another major trend in plate making. WO 90002044, WO 91008108 and EP-A 580 394 disclose such plates, which are all ablative plates. These methods require typically multi-layer materials, which makes them less suitable for on-press coating. A non-ablative plate which can be processed with plain water is described in e.g. EP-A 770 497 and EP-A 773 112. Such plates also allow on-press processing, either by wiping the exposed plate with water while being mounted on the press or by the fountain solution during the first runs of the printing job.
(iv) Thermal imaging. Most of the computer-to-press methods referred to above use so-called heat-mode materials, i.e. plate precursors or on-press coatable compositions which comprise a compound that converts absorbed light into heat. The heat which is generated on image-wise exposure triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex. In addition to some of the disadvantages of the prior art materials and methods, indicated above, a major problem associated with all the known non-ablative thermal materials is the limited shelf life. Because these materials all contain one or more reactive compounds, the stability is highly dependent on temperature and/or humidity conditions during storage.
Most high quality printing jobs require the use of a printing master which comprises a metal support characterized by a good dimensional stability, e.g. a printing plate comprising an aluminum support. Since the metal support is an expensive component of the printing master, it would be advantageous to provide a method for removing the printing surface from the support so that it can be reused for making another printing master. Also in on-press coating methods it is necessary to remove the coating from the cylinder by a cleaning step. Such a cleaning step is preferably carried out on-press by a fast, environment-friendly and simple operation which enables easy automatization, thereby obtaining a more efficient workflow which is characterized by improved convenience and a short press down-time between printing jobs.
Methods which have been proposed in the prior art for erasing a printing master typically involve mechanical cleaning by scraping, chipping or rubbing, e.g. in U.S. Pat. No. 5,188,033. However, such abrasive methods may damage the metal support. EP-A 594 097 describes the use of solvents for removing the printing layers from a metal support. Solvents however are secondary wastes which need to be collected during the cleaning step and then disposed off.
EP-A 570 879 describes a method using a jet of pressurized water for erasing a lithographic printing master obtained by image-wise thermal transfer of an ink accepting layer. In addition to the problem of collecting the water, as in the methods using a solvent mentioned above, the use of water is to be avoided as it may cause corrosion of the metal parts of the printing press.
U.S. Pat. No. 5,317,970 describes the use of a plasma treatment to erase a lithographic printing master. However, the latter method is not convenient because it involves the use of a high-vacuum plasma chamber. Moreover, it is mentioned in the latter patent that plasma treatment at atmospheric pressure is only possible at very high temperatures which results in a damaged printing form.
It is an object of the present invention to provide an improved method for removing ink accepting areas of the printing surface of a lithographic printing master by a fast, environment-friendly, convenient and simple cleaning step which enables easy automatization. More specifically, it is an object of the present invention to provide a cleaning method which is not abrasive, which produces no secondary wastes such as solvents, which does not use corrosive liquids such as water or hazardous chemicals and which can be used at ambient temperature and atmospheric pressured;
It is another object of the present invention to use such a method for cleaning a reusable support of an imaging material which is suitable for making a lithographic printing master, thereby obtaining a more efficient workflow which is characterized by improved convenience and a short press down-time between printing jobs.
Further objects and advantages of the present invention will become apparent from the following description. These objects and advantages are obtained by preferred embodiments of the present invention which are specified in the dependent claims.